Improved SIMD test data to use all bits

Improved SIMD test data to use all bits

The SIMD test data now uses all available bits in the
current precision floating-point numbers, to catch errors
where we lose precision or have mistakes in single/double
conversions (e.g. return values). Since some operations now
depend on the least significant bit, a few tests have been
relaxed to allow a tolerance rather than require exact
binary matching.

The new tests caught a precision loss in a reduction
function for the SIMD-mimicking scalar functions, but this
has never been used apart from testing, and should be harmless.

Fixes #2163.

Change-Id: I6d8f19d2aafeee8f42a2034c6100fcb10f6d2e81